This invention relates to a soldering method that employs a preformed solder body composed of a tin-lead alloy, for example, in attachment of an integrated circuit component to a printed circuit board. More particularly, this invention relates to such method using a tin-lead solder body that includes a palladium deposit to enhance wetting of the substrate metal and thereby produce a strong metallurgical bond even in the absence of rosin flux.
In the manufacture of an electrical component package, it is common practice to assemble components by solder bump interconnections formed from microballs of solder alloy. For example, an integrated circuit chip may be mounted onto a ceramic carrier by an array of solder bumps. Each bump is bonded to a metallic terminal on the chip and also bonded to a metallic terminal on the carrier. In this manner, the solder bumps not only physically attach the components, but also electrically interconnect the terminals to permit electrical signals to be conducted to and from the integrated circuit chip for processing.
Each solder bump interconnection is formed from a singular microsphere of a suitable lead-tin alloy. A solder ball is heated in contact with a terminal of a first component, typically the chip terminal, to reflow the solder alloy onto the terminal metal and, upon cooling, form a solder bump bonded to the terminal. This first component is arranged with the second component with each said bump resting upon a terminal of the second component. In this arrangement, the components are briefly heated to reflow the solder alloy and bond the bump to the second terminal, thereby joining the components into a product assembly.
During each reflow, the liquid solder alloy wets the metal surface of the terminal to provide intimate contact that is necessary to obtain a strong metallurgical bond. To promote wetting of the terminal, a flux is applied to the interface between the solder alloy and the terminal prior to reflow. Rosin flux is particularly well suited to promote wetting, but leaves a residue on the surfaces of the assembly that may interfere with subsequent processing steps or may accelerate corrosion of the solder metal. Accordingly, rosin-fluxed soldering operations are followed by cleaning the assembly with a chlorinated solvent, such as trichloroethylene. This cleaning is complicated by the need to control emission of solvent vapors. Thus, it is desired to eliminate the use of rosin flux and to avoid the subsequent solvent cleaning operation, but without sacrificing intimate wetting of the substrate metal by the solder alloy that is essential to formation of a strong metallurgical bond therebetween.